Circuit and Method for Detecting the Duration of the Interruption of a Mains Input

ABSTRACT

A power supply or driver circuit configured to provide electrical energy at a drive voltage. The driver circuit converts electrical energy at an input voltage to the electrical energy at the drive voltage. A controller is configured to control the power converter to provide electrical energy at the drive voltage. The controller stops operation at an interruption of electrical energy to the driver circuit. The controller is configured to resume operation subsequent restoration of electrical energy to the driver circuit. The controller is configured to maintain the timing voltage above a first voltage level when the controller is in operation and to determine the duration of an interruption of electrical energy to the driver circuit.

TECHNICAL FIELD

The present document relates to a power supply or driver circuit. Inparticular, the present document relates to a power supply or drivercircuit for a light bulb assembly.

BACKGROUND

A power supply, e.g. a power supply for a light bulb assembly, shouldexhibit short start-up times, in order to allow for a quick start-up ofthe device coupled to the power supply, e.g. in order to allow for aquick start-up of the light bulb assembly. The power supply may beconfigured to convert electrical energy from a mains supply toelectrical energy for a light source of a light bulb assembly at apre-determined voltage level. Examples for light sources are lightemitting diodes (LED) and/or organic LEDs (OLEDs), which are typicallyreferred to as solid state lighting (SSL) devices. The power supply mayalso be referred to as a driver circuit.

Light bulb assemblies may be dimmable subject to control events receivedvia the mains supply. Typically, phase-cut dimmers are used to cut aphase of the waveform of the current provided by the mains supply to thelight bulb assembly. The phase-cut angle is typically indicative of thedegree of dimming to be provided by the light bulb assembly. In thepresent document, a method for enabling dimming without the need of aphase-cut dimmer is described. This method makes use of briefinterruptions of the mains supply which may be created by a brief off/oncycle using a mains power switch. Such a brief interruption of the mainssupply may be interpreted by a controller comprised within the drivercircuit of the light bulb assembly as an instruction to dim the light.

It is expected that the dimming of SSL based light bulb assemblies inresponse to events triggered by such off/on cycles of the mains powerswitch will become an important topic in the near future, as thistechnique allows the implementation of dimming without the need for aphase-cut dimmer. In order to enable a light bulb assembly to detectsuch off/on cycles, the controller of the driver circuit of the lightbulb assembly may be provided with a counter for measuring the length ofan OFF period, i.e. the period during which the mains supply is off. Bydoing this, the controller can distinguish between an off/on cycle(having a pre-determined maximum duration of the OFF period) fortriggering the dimming of the light bulb assembly, and an intentional“turn-off” of the light bulb assembly (if the duration of the OFF periodexceeds the pre-determined maximum duration).

If a timer or counter within the controller is used to measure theduration of an OFF period, the controller needs to maintain activeoperation even during time windows with no mains supply. As aconsequence, relatively large storage capacitors may be required, inorder to keep the controller running during the OFF period. However, theuse of additional storage capacitors adds costs and space to the drivercircuit. Furthermore, the use of additional and/or large storagecapacitors may have a negative impact on the lifetime of the light bulbassembly, because such storage capacitors are typically AluminumElectrolyte type capacitors which exhibit reduced lifetimes. Inaddition, the use of large storage capacitors (e.g. large supply voltagecapacitors) may have a negative impact on the start-up time of thedriver circuit and of the light bulb assembly.

The present document addresses the above mentioned technical problemsand describes a driver circuit and/or power supply which enables the useof relatively small supply voltage capacitors (for short start-up times,reduced costs, reduced space and increased lifetime), while at the sametime enabling the determination of the duration of an OFF period (forproviding a light bulb assembly which is dimmable subject to off/onevents of the mains supply).

SUMMARY

According to an aspect, a driver circuit or a power supply, e.g. for alight bulb assembly, is described. The driver circuit is configured toprovide, at an output of the driver circuit, electrical energy at adrive voltage. The drive voltage may correspond to the on-voltage of alight source (e.g. an SSL based light source) of the light bulbassembly. The electrical energy at the drive voltage may be derived fromelectrical energy at an input voltage at an input of the driver circuit.The electrical energy at the input voltage may correspond to theelectrical energy provided by a mains supply. In particular, the inputvoltage may correspond to a rectified mains voltage. Alternatively or inaddition, the input voltage may correspond to a voltage provided by atransformer, e.g. for an halogen bulb. As such, the driver circuit (andthe light bulb assembly) may be used for a replacement of halogen lightbulbs.

The driver circuit may comprise a power converter configured to convertthe electrical energy at the input voltage into the electrical energy atthe drive voltage. By way of example, the power converter may comprise aswitched-mode power converter, such as a boost converter, a buckconverter, a-buck-boost converter, a flyback converter and/or a SEPIC(Single-ended primary-inductor converter) converter. The power convertermay comprise a power switch (e.g. a transistor such as a metal oxidesemiconductor field effect transistor).

Furthermore, the driver circuit may comprise a controller (which is e.g.implemented as an integrated circuit). The controller may be configuredto control the power converter (e.g. the power switch of the powerconverter) such that the power converter provides the electrical energyat the drive voltage. The controller is typically configured to stopoperation subsequent to (and/or as a result of) an interruption ofelectrical energy provided at the input of the driver circuit (e.g. dueto an interruption of the mains supply). Furthermore, the controller istypically configured to resume operation subsequent to (and/or as aresult of) the provision of electrical energy at the input of the drivercircuit (e.g. due to the provision of electrical energy by the mainssupply).

The driver circuit may comprise a supply voltage capacitor which may be(directly) coupled to the controller and which may be configured toprovide a supply voltage to the controller. The supply voltage capacitormay be charged using electrical energy provided at the input of thedriver circuit (e.g. using the electrical energy provided by the mainssupply). Subsequent to the interruption of the electrical energyprovided at the input of the driver circuit, the supply voltage maydrop. In particular, the interruption of the electrical energy may havethe effect that the supply voltage capacitor is not recharged anymore.At the same time, the controller typically continues to draw a currentfrom the supply voltage capacitor, thereby discharging the supplyvoltage capacitor. As a result, the supply voltage provided by thesupply voltage capacitor drops. The controller is typically configuredto stop operation, if the supply voltage is below a pre-determinedsupply voltage threshold. On the other hand, the controller is typicallyconfigured to resume operation, if the supply voltage is above thesupply voltage threshold.

The driver circuit comprises a timing circuit which may be (directly)coupled to the controller. In particular, the timing circuit may couplea pin of the controller to ground (or to another pre-determined voltagelevel). The timing voltage at the timing circuit decays with apre-determined time constant, if not maintained. By way of example, thetiming circuit may comprise a capacitor. The capacitor may be arrangedin parallel to a resistor. The timing voltage may correspond to thevoltage drop across the capacitor and/or across the resistor. If thecapacitor of the timing circuit is not recharged to maintain the timingvoltage at a pre-determined level (e.g. at or above a first voltagelevel), the timing voltage may decay with the pre-determined timeconstant.

The controller may be configured to maintain the timing voltage at orabove the first voltage level (also referred to as the on-voltagethreshold), when the controller is in operation. In particular, thecontroller may be configured to couple the timing circuit to a voltagesource to maintain the timing voltage at or above the first voltagelevel. This may be achieved by one or more power switches comprisedwithin the controller, which are configured to recharge the capacitor ofthe timing circuit using the electric energy provided at the input ofthe driver circuit. Furthermore, the controller may be configured tointerrupt the maintenance of the timing voltage, when the controller hasstopped operation. In particular, the controller may be configured tostop the recharging of the capacitor, once the controller has stoppedoperation.

Furthermore, the controller may be configured to determine an indicationof a residual level of the timing voltage when the controller resumesoperation. In particular, upon resumption of operation, the controllermay be configured to first determine the indication of the residuallevel of the timing voltage and to then bring back the timing voltage upto the first voltage level (e.g. by recharging the capacitor of thetiming circuit). The indication of the residual level of the timingvoltage may e.g. comprise (or correspond to) an indication on whetherthe residual level of the timing voltage is greater than or smaller thana pre-determined second voltage level (referred to herein as thelow-voltage threshold).

In addition, the controller may be configured to determine an indicationof the duration of an interruption of electrical energy provided at theinput of the driver circuit. The indication of the duration may bedetermined based on the indication of the residual level of the timingvoltage. The indication of the duration may comprise (or may correspond)to an indication on whether the duration is greater than or smaller thana pre-determined duration threshold (corresponding e.g. to apre-determined number of cycles of the mains voltage). The controllermay further take into account the first voltage level and thepre-determined time constant for determining the indication of theduration of the interruption of electrical energy provided at the inputof the driver circuit;

The controller may be configured to couple the timing circuit with acomparator (comprising e.g. an operational amplifier). The comparatormay be implemented within the controller. The comparator may beconfigured to compare the timing voltage with a reference voltage (alsoreferred to as the second voltage level or as the low voltagethreshold). The reference voltage may depend on the pre-determined timeconstant of the timing circuit. Furthermore, the reference voltage maydepend on the first voltage level. In addition, the reference voltagemay depend on the pre-determined duration threshold. In particular, thereference voltage may be selected such that a residual value of thetiming voltage which lies above the reference voltage indicates aduration of the interruption which is below the pre-determined durationthreshold and such that a residual value of the timing voltage whichlies below the reference voltage indicates a duration of theinterruption which is above the pre-determined duration threshold. Thecomparator may be configured to indicate whether the timing voltage isgreater or smaller than the reference voltage, thereby providing anindication of the residual level of the timing voltage. As such, thecomparator may be configured to indicate whether the duration of theinterruption is greater than or smaller than the pre-determined durationthreshold.

By using a timing circuit which is separate from the controller, it maybe achieved to determine (an indication of) the duration of theinterruption of electrical energy provided at the input of the drivercircuit (e.g. the interruption of the mains supply), without the need ofincreasing the capacitance of the supply voltage capacitor, i.e. withoutthe need of increasing the start-up time of the driver circuit.

As indicated above, the power converter may comprise a power switch. Thepower switch may be arranged in series with a shunt resistor (whereinthe shunt resistor may link the source of the power switch directly toground). The shunt resistor may be arranged such that a voltage drop atthe shunt resistor is indicative of a current through the power switch.The timing circuit may couple the controller to the shunt resistor (viaa pin of the controller). As such, the pin of the controller may be usedfor measuring the current through the power switch (during normaloperation of the controller) and for measuring the residual value of thetiming voltage (subsequent to an interruption of operation of thecontroller). As such, no additional pin is needed at the controller toperform the measurement of the residual value of the timing voltage.

The controller may be configured to couple the timing circuit with thecomparator via a compensation capacitor. The compensation capacitor maybe configured to compensate the timing voltage such that the voltage atthe comparator corresponds to the voltage drop at the shunt resistor.The driver circuit may further comprise an auto zeroing switchconfigured to couple the compensation capacitor to a pre-determinedpotential, when charging the compensation capacitor. The auto zeroingswitch may ensure that the voltage drop across the compensationcapacitor corresponds to the inverse of the timing voltage. By doingthis, it can be ensured that the voltage at the input of the comparatorcorresponds to the voltage drop at the shunt resistor, thereby allowingthe comparator to measure the current through the power switch duringnormal operation of the controller.

The driver circuit may be configured to provide the electrical energy atthe drive voltage to a solid state lighting light source (e.g. to aplurality of LEDs or OLEDs). The controller may be configured to operatethe power converter according to a plurality of different operationstates corresponding to a plurality of different illumination states ofthe light source. Examples for illumination states are e.g. an “off”state with no illumination, an “on” state with a maximum illuminationlevel, a “dim up” state with a smoothly increasing illumination leveland a “dim down” state with a smoothly decreasing illumination level.

In particular, the controller may be configured to operate the drivercircuit according to a current operation state and to detect one of aplurality of pre-determined events based on the input voltage. Theplurality of pre-determined events may comprise an “off” event, an“off/on” event and an “on” event. Detecting the “off” event may comprisedetecting that the input voltage lies below a pre-determined first inputvoltage threshold for more than the pre-determined duration threshold.Detecting the “off/on” event may comprise detecting that the inputvoltage lies below the pre-determined first input voltage threshold forless than the pre-determined duration threshold. Detecting the “on”event may comprise detecting that the input voltage lies above apre-determined second input voltage threshold. The first input voltagethreshold may be lower than the second input voltage threshold.

The controller may be configured to determine a target operation statein accordance to a pre-determined state machine, based on the currentoperation state and based on the detected one of the plurality ofpre-determined events. The pre-determined state machine may definetransitions between different operation states subject to detectedevents. Subsequent to determining the target operation state, thecontroller may be configured to operate the driver circuit in accordanceto the target operation state (e.g. for dimming the light source).

The driver circuit may further comprise an output capacitor at theoutput of the driver circuit. The output capacitor may be configured tostore an electrical charge to be provided to a load (e.g. the lightsource) of the driver circuit. The output capacitor may be arranged inparallel to the load. The driver circuit may be configured to transferelectrical energy from an inductor of the power converter to the outputcapacitor during an off-state of the power switch. The electrical chargeof the output capacitor may be used to provide electrical energy to theload (e.g. during an interruption of the input voltage).

According to a further aspect, a light bulb assembly is described. Thelight bulb assembly may comprise an electrical connection moduleconfigured to electrically connect to a mains power supply, therebyproviding electrical energy at the input voltage. Furthermore, the lightbulb assembly may comprise a driver circuit as described in the presentdocument. The driver circuit may be configured to provide electricalenergy at the drive voltage derived from the electrical energy at theinput voltage. Furthermore, the light bulb assembly comprises a lightsource (e.g. a series of LEDs or OLEDs) configured to provide lightusing the electrical energy at the drive voltage.

According to another aspect, a method for operating a controller and/ora driver circuit as outlined in the present document is described. Themethod may comprise steps which correspond to the features of thecontroller and/or driver circuit described in the present document. Inparticular, a method for determining an indication of the duration of aninterruption of electrical energy provided to a driver circuit isdescribed. The method may comprise controlling a power converter toconvert electrical energy at an input voltage into electrical-energy ata drive voltage using a controller. Furthermore, the method may comprisemaintaining a timing voltage at a timing circuit above a first voltagelevel, when controlling the power converter. The timing circuit isseparate from the controller and the timing voltage at the timingcircuit decays with a pre-determined time constant if not maintained.The method may comprise stopping controlling the power converter andstopping maintaining the timing voltage, subsequent to an interruptionof electrical energy provided to the driver circuit. In addition, themethod may comprise resuming controlling the power converter, subsequentto the provision of electrical energy to the driver circuit. Anindication of a residual level of the timing voltage may be determinedwhen resuming controlling the power converter. The indication of theduration of the interruption of electrical energy provided to the drivercircuit may be determined based on the first voltage level, based on thepre-determined time constant and/or based on the indication of theresidual level of the timing voltage.

According to a further aspect, a software program is described. Thesoftware program may be adapted for execution on a processor and forperforming the method steps outlined in the present document whencarried out on the processor.

According to another aspect, a storage medium is described. The storagemedium may comprise a software program adapted for execution on aprocessor and for performing the method steps outlined in the presentdocument when carried out on the processor.

According to a further aspect, a computer program product is described.The computer program may comprise executable instructions for performingthe method steps outlined in the present document when executed on acomputer.

It should be noted that the methods and systems including its preferredembodiments as outlined in the present document may be used stand-aloneor in combination with the other methods and systems disclosed in thisdocument. In addition, the features outlined in the context of a systemare also applicable to a corresponding method. Furthermore, all aspectsof the methods and systems outlined in the present document may bearbitrarily combined. In particular, the features of the claims may becombined with one another in an arbitrary manner.

In the present document, the term “couple” or “coupled” refers toelements being in electrical communication with each other, whetherdirectly connected e.g., via wires, or in some other manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below in an exemplary manner with referenceto the accompanying drawings.

FIG. 1 illustrates a block diagram of an example light bulb assembly.

FIG. 2 shows a circuit diagram of an example driver circuit.

FIG. 3 illustrates an example method for enabling dimming of a lightbulb assembly using a mains switch.

FIG. 4 a shows a block diagram of an example circuit arrangementconfigured to measure the duration of an OFF period of the mains supply.

FIG. 4 b shows the voltages at the timing circuit as a function of time.

FIGS. 5 a and 5 b show circuit diagrams of example circuit arrangementsconfigured to measure the duration of an OFF period of the mains supply.

FIG. 6 illustrates a flow chart of an example method for providing anindication of the duration of an OFF period of the mains supply.

DESCRIPTION

In the present document, a light bulb “assembly” includes all of thecomponents required to replace a traditional incandescent filament-basedlight bulb, notably light bulbs for connection to the standardelectricity supply. In British English (and in the present document),this electricity supply is referred to as “mains” electricity, whilst inUS English, this supply is typically referred to as power line. Otherterms include AC power, line power, domestic power and grid power. It isto be understood that these terms are readily interchangeable, and carrythe same meaning.

Typically, in Europe electricity is supplied at 230-240 VAC, at 50 Hz(mains frequency) and in North America at 110-120 VAC at 60 Hz (mainsfrequency). The principles set out in the present document apply to anysuitable electricity supply, including the mains/power line mentioned,and a DC power supply, and a rectified AC power supply.

FIG. 1 is a schematic view of a light bulb assembly. The assembly 1comprises a bulb housing 2 and an electrical connection module 4. Theelectrical connection module 4 can be of a screw type or of a bayonettype, or of any other suitable connection to a light bulb socket.Typical examples for an electrical connection module 4 are the E11, E14and E27 screw types of Europe and the E12, E17 and E26 screw types ofNorth America. Furthermore, a light source 6 (also referred to as anilluminant) is provided within the housing 2. Examples for such lightsources 6 are a CFL tube or a solid state light source 6, such as alight emitting diode (LED) or an organic light emitting diode (OLED)(the latter technology is referred to as solid state lighting, SSL). Thelight source 6 may be provided by a single light emitting diode, or by aplurality of LEDs.

Driver circuit 8 is located within the bulb housing 2, and serves toconvert supply electricity received through the electrical connectionmodule 4 into a controlled drive current for the light source 6. In thecase of a solid state light source 6, the driver circuit 8 is configuredto provide a controlled direct drive current to the light source 6.

The housing 2 provides a suitably robust enclosure for the light sourceand drive components, and includes optical elements that may be requiredfor providing the desired output light from the assembly. The housing 2may also provide a heat-sink capability, since management of thetemperature of the light source may be important in maximising lightoutput and light source life. Accordingly, the housing is typicallydesigned to enable heat generated by the light source to be conductedaway from the light source, and out of the assembly as a whole.

The driver circuit 8 of a light bulb assembly 1 should be configured toprovide a drive current to the light source 6 almost instantaneously,subsequent to turning on of the mains supply (e.g. subsequent to a userswitching on the light). Consequently, the driver circuit 8 shouldexhibit a low start-up time. On the other hand, the driver circuit 8should be configured to measure the duration of short interruptions ofthe mains supply which may be used to control the behaviour of the lightbulb assembly 1. By way of example, intentional interruptions of themains supply may enable a user to control the dimming of the light bulbassembly 1 using an on/off light switch. The driver circuit 8 should beconfigured to determine the duration of such interruptions withoutimpacting the start-up time of the driver circuit 8.

In the following, an example driver circuit 8 for a light bulb assembly1 is described in more detail. In particular, a driver circuit 8 isdescribed which may be used to control the dimming level of the lightsource 6 of the light bulb assembly 1, subject to intentionalinterruptions of the mains supply.

FIG. 2 illustrates a circuit diagram of an example driver circuit 200, 8of a light bulb assembly 1. The driver circuit 200 comprises anelectromagnetic interference (EMI) filter unit 201 and a rectifier 202,in order to generate a rectified voltage from the main supply 230. Itshould be noted that the voltage 230 may be referred to in general termsas an input voltage 230. The input voltage 230 may correspond to thevoltage provided by a transformer (e.g. 12V or 24V), e.g. for an halogenlight bulb. The transformer may e.g. comprise a 50 Hz transformer and/oran electronic transformer. Furthermore, the driver circuit 200 comprisesa controller 206 which is configured to control one or more powerswitches 211, 212 of one or more switched-mode power converters 221,222. The current through a power switch 212 may be sensed at a pin 216of the controller 206 using a shunt resistor 215. As such, the shuntresistor 215 may be viewed as current sensing means for sensing thecurrent through the power switch 212. The controller 206 may use thesensed current through the power switch 212 for controlling theswitching time instant of the power switch 212.

The controller 206 may be started using the start-up resistor 205. Thestart-up resistor 205 may be coupled via an internal connection withinthe controller 206 to the supply voltage capacitor 203 of the controller206, thereby enabling the charging of the supply voltage capacitor 203.Once the supply voltage capacitor 203 has reached a sufficient charginglevel, the controller 206 can start operation. The charge of the supplyvoltage capacitor 203 may be used to maintain the supply voltage for thecontroller 206 at a pre-determined level, and to bridge briefinterruptions of the mains supply. The duration of the interruptionswhich may be bridged depend on the capacitance of the supply voltagecapacitor 203. If the charge of the supply voltage capacitor 203 dropsbelow a pre-determined level, i.e. if the voltage drop at the supplyvoltage capacitor 203 drops below a pre-determined voltage level (e.g.due to an interruption of the mains supply), the controller 203 stopsoperation.

In the illustrated example, the driver circuit 200 comprises a two-stagepower converter 221, 222 with the first stage 221 being a Boostconverter and the second stage 222 being a flyback converter. Theflyback converter comprises a transformer 207 having a primary winding214, a secondary winding and an additional auxiliary winding 213. Theauxiliary winding 213 may be used to provide information to thecontroller 206 regarding the output voltage 231 (also referred to as thedrive voltage) of the driver circuit 200. Furthermore, the drivercircuit 200 comprises an output capacitor (or storage capacitor) 208which stores electrical energy to be provided to the light source 6,209. The driver circuit 200 is configured to provide the light source209 with the drive voltage 231 (which corresponds e.g. to the on-voltageof an LED device) and with a drive current 232 (which may be controlledin order to control the illumination level of the light source 209).

The driver circuit 200 (and in particular the controller 206) may beconfigured to detect one or more events encoded within the mains voltage230. By way of example, the mains voltage 230 may be submitted to amains switch (e.g. an on/off switch) configured to provide and tointerrupt the mains voltage 230. A brief intentional interruption of themains voltage 230 (e.g. having a duration which exceeds a minimum numberof cycles of the mains supply 230 and which is smaller than a maximumnumber of cycles of the mains supply 230) may be interpreted by thecontroller 206 as a so called “Off/On event”. Furthermore, thecontroller may be configured to detect an “On event”, subject to themains switch being turned on, thereby providing the mains voltage 230.In addition, the controller may be configured to detect an “Off event”,subject to an interruption of the mains voltage having a duration whichexceeds the above mentioned maximum number of cycles of the mains supply230.

The controller 206 may comprise a state machine (e.g. encoded assoftware, firmware or hardware). The state machine may define aplurality of illumination states of the light source 209, as well asevents which trigger transitions between the illumination states. By wayof example, the plurality of illumination states may comprise a “Hold”illumination state at a fixed illumination level, a “Max” illuminationstate at a maximum illumination level, an “Off” illumination state atzero illumination level, a “dim up” illumination state (where theillumination level is smoothly increased) and/or a “dim down”illumination state (where the illumination level is smoothly decreased).

The controller 206 may be configured to determine a target illuminationstate of the light source 209 starting from a current illuminationstate, in accordance to the pre-determined state machine. In particular,the controller 206 may be configured to define and adjust settings ofthe driver circuit 200 (e.g. the duty cycle of a power switch 211, 212),which control the respective behavior of the light source 209 for thecorresponding illumination states. The settings of the driver circuit200 (e.g. the length of the duty cycle of a power switch 211, 212) maybe stored in a memory of the controller 206.

FIG. 3 illustrates a method 300 which makes use of a mains switch 302 assignaling means, in order to encode a plurality of events 310. Theexample events of FIG. 3 are an “ON” event, corresponding to a switch302 which is kept on for a minimum pre-determined time interval; an “OFFevent, corresponding to a switch 302 which is kept off for a minimumpre-determined time interval; and an “OFF/ON” event, corresponding to anevent where the switch 302 is briefly switched from ON to OFF and thenback to ON within a predetermined time interval. These three events incombination with various illumination states can be used to provide adimming function (even when no phase-cut dimmer is available at themains supply).

It can be seen from FIG. 3, how the ON event 312 can be used to changethe driver circuit 200 from the state “OFF”, i.e. no intensity,(reference numeral 322) to the state “MAX”, i.e. maximum intensity,(reference numeral 321), and how the OFF event 311 can be used toperform the inverse change of states. When in the “MAX” state 321, theOFF/ON event 313 can be used to put the driver circuit 200 into a dimdown state 323. The dim down state 323 triggers a smooth decrease inintensity down towards a minimum intensity (e.g. 20% intensity). Usinganother OFF/ON event 314, the dim down state 323 may be stopped, therebyputting the driver circuit 200 into a hold state 324 at the currentintensity. When detecting another OFF/ON event 316, the driver circuit200 is put into a dim up state 325, thereby smoothly increasing theintensity up to the maximum intensity. Another OFF/ON event 317 mayagain put the driver circuit 200 into a hold state 326 (which differsfrom the hold state 324 in that a subsequent OFF/ON event 318 willretrigger the dim down state 323, instead of the dim up state 325).Whenever detecting an OFF event 315, 319, the driver circuit 200 is putinto the OFF state 322. In the example method 300 (and the correspondingexample state machine of the controller 206), this OFF state 322 canonly be left, when detecting an ON event 312.

In order to provide light bulb assemblies 1 which may be dimmed usingthe control method 300 illustrated in FIG. 3, the driver circuit 200should be configured to determine the duration of interruptions of themains supply 230 (which are used to signal an “Off/on” event). Thiscould be achieved by using a counter within the controller 206. This,however, requires the controller 206 to maintain operation withoutreceiving energy from the mains supply. The operation of the controller206 may be maintained using a supply voltage capacitor 203 having anincreased capacity, thereby extending the duration during which thesupply voltage capacitor 203 can supply power to the controller 206.Typically, the controller 206 draws a current in the range of 50 μA.Consequently, a relatively large supply voltage capacitor 203 isrequired if an interruption of about 1 or 2 seconds duration is to bebridged and the duration of the interruption is to be determined.However, a large supply voltage capacitor 203 leads to increasedstart-up times which is in contradiction with the design requirement ofproviding a light bulb assembly 1 which emits light almostinstantaneously, in response to turning on of the mains switch 302.

In the present document it is proposed to use a dedicated circuitarrangement for measuring the duration of an interruption of the mainssupply (referred to as an “OFF” period), wherein the dedicated circuitarrangement is separate from the controller 206. The dedicated circuitarrangement may be configured to provide an indication of the durationof an interruption of the mains supply, even if the controller 206 hasstopped operation due to the interruption of the mains supply. Thecircuit arrangement may be referred to as a timing circuit exhibiting atime constant. By way of example, the timing circuit may comprise an RCcircuit exhibiting the time constant given by τ=RC, wherein R is theresistance of a resistor and wherein C is the capacitance of a capacitorof the RC circuit.

FIG. 4 a shows a circuit arrangement 400 comprising an example timingcircuit 401. The timing circuit 401 comprises an RC circuit with acapacitor 402 arranged in parallel with a resistor 403. The timingcircuit 401 may be coupled to the controller 206 via a pin 216. Thecontroller 206 and the timing circuit 401 may be part of the drivercircuit 200 of a light bulb assembly 1. During normal operation of thecontroller 206 (i.e. during “ON” periods of the mains supply, when themains switch 302 is switched on), the controller 206 may be configuredto maintain the voltage 422 at the timing circuit 401 at apre-determined level. This may be achieved e.g. using switches 404 and405 of the controller 206. On the other hand, when the controller 206stops operation (e.g. subject to an “OFF” period of the mains supply,triggered by the mains switch 302 being switched off), the timingcircuit 401 may not be recharged anymore. As a result, the voltage 422at the timing circuit 401 may decrease in accordance to the timeconstant τ of the timing circuit 401. This is illustrated in FIG. 4 bwhere the voltage 422 at the timing circuit 401 is shown as a functionof time 421. It can be seen that during operation of the controller 206,the voltage 422 at the timing circuit 401 is maintained at or above apre-determined on-voltage threshold 423. When the controller 206 stopsoperation, the voltage 422 at the timing circuit 401 drops in accordanceto the time constant τ. As soon as the controller 206 resumes operation(e.g. at time instant 426), the voltage 422 at the timing circuit 401 isbrought back up to or above the on-voltage threshold 423. On the otherhand, if the controller 206 does not resume operation for a certain timeperiod, the voltage 422 at the timing circuit 401 drops below apre-determined off-voltage threshold 424 (e.g. at time instant 427).

The controller 206 may be configured to measure the voltage 422 at thetiming circuit 401 when resuming operation (e.g. at time instant 426)prior to bringing back up the voltage 422 at the timing circuit 401(e.g. prior to recharging the capacitor 402 of the timing circuit 401).The voltage 422 at the timing circuit 401 may be referred to as thetiming voltage 422. The timing voltage 422 may be measured prior torecharging of the timing circuit 401 using the switches 404, 405. Thecontroller 206 may be configured to couple the timing circuit 401 viathe pin 216 to a comparator 411 (e.g. using an internal switch 406). Thecomparator 411 may be configured to compare the timing voltage 422 attime instant 426 with a reference voltage (wherein the reference voltagemay be provided by the capacitor 410 which is charged via the switch409). The reference voltage may e.g. correspond to the off-voltagethreshold 424.

As such, the comparator 411 may be configured to determine whether thetiming voltage 422 at the time instant 426 is greater or smaller thanthe off-voltage threshold 424. In view of the pre-determined timeconstant τ of the timing circuit 401 (which defines the speed of decayof the timing voltage 422), the off-voltage threshold 424 is directlyrelated to the duration 425 of an OFF period. By way of example, thetime constant τ may define the duration for the timing voltage 422 todrop from a value V₀ to a value V₀/e, wherein e is Euler's number.Consequently, the duration of an OFF period can be determined knowingthe initial value V₀ of the timing voltage 422, knowing the timeconstant τ and knowing the value of the timing voltage 422 at the timeinstant 426 when the controller 206 resumes operation. In particular, itmay be determined whether the duration of the OFF period is above orbelow a pre-determined duration threshold (which corresponds e.g. to apre-determined number of cycles of the mains supply) by determiningwhether the timing voltage 422 at the time instant 426 is above or belowthe pre-determined off-voltage threshold 424.

The on-voltage threshold 423 and the off-voltage threshold 424 (as wellas the time constant τ) may be selected based on the above mentionedmaximum number of cycles of the mains supply 230, which correspond tothe maximum duration of an OFF period, which is interpreted by thecontroller 206 to be an “Off/On” event. The comparator 411 is configuredto determine whether the timing voltage 422 at the end 426 of an OFFperiod is below or above the off-voltage threshold 424. If the timingvoltage 422 is above the off-voltage threshold 424, the controller 206may detect an “Off/On” event. On the other hand, if the timing voltage422 is below the off-voltage 424, the controller 206 may detect an “Off”event.

The circuit arrangement 400 may make use of a biasing circuit 407, 408in order to adjust the measured timing voltage 422 to different levels.

Hence, an external analog R/C timing element 401 may be used togetherwith an intelligent charge/detect circuit 411 in order to measure theduration of a mains switch off/on window and in order to generatetrigger events for various dimming actions.

For the switch dim function the latest dimming information has storedover a time of about 2 seconds including a function of time out. Anexternal RC combination can be used for this function. By way ofexample, as shown in FIG. 5 a, the circuit arrangement 500 may comprisea data storage 504 (which may e.g. comprise a Random Access Memory). Thedata storage 504 may be implemented within the controller 206. The datastorage 504 may be configured to store a current state of the light bulbassembly 1 (e.g. a current illumination level of the light source 209).The timing circuit 401 may be configured to supply energy to the datastorage 504 for a pre-determined amount of time (e.g. for an amount oftime which corresponds to the pre-determined duration threshold used todetect an OFF/ON event). As such, the data storage 504 may be configuredto store the current state of the light source 209 for a duration whichis sufficiently long to bridge an interruption of the mains supplyhaving a duration which corresponds to the duration of an OFF/ON event.By doing this, the state machine of the controller 206 may determine thetarget state of the light source 209 based on the current state of thelight source 209 stored in the data storage 504.

On the other hand, if the duration of the interruption exceeds thepre-determined amount of time (e.g. the pre-determined durationthreshold), the voltage at the data storage 504 drops below apre-determined threshold (e.g. below the low voltage threshold 424) suchthat the data storage 504 may be reset. The light bulb assembly 1 maythen start operation from a default starting state (e.g. the MAXillumination state).

FIG. 5 a shows a further circuit arrangement 500 comprising a controller206 and a timing circuit 401. The timing circuit 401 is coupled on afirst side of the timing circuit 401 to ground and on another side ofthe timing circuit to a pin 216 of the controller 206. The timingvoltage 422 corresponds to the voltage drop across the timing circuit401 between the pin 216 of the controller 206 and ground. During normaloperation, the capacitor 402 of the timing circuit 401 may be charged toan initial voltage V₀ using the voltage source 403 and the switch 502.The initial voltage V₀ is typically at or above the on-voltage threshold424. If the power fails (e.g. when the controller 206 stops operation orwhen an interruption of the mains supply is detected), the capacitor 402will not be re-charged anymore and the capacitor 402 is discharged viathe resistor 403, wherein the timing voltage 422 is reduced inaccordance to the time constant given by τ=RC. The voltage will be highenough, that the data will be stored. As outlined above, the datastorage 504 may be configured to store the current illumination state ofthe light source 209 (e.g. the current dim level). The data storage 504(e.g. a RAM) may require a pre-determined minimum voltage level foroperation. The voltage for the data storage 504 may be provided by thetiming circuit 401. If the voltage level drops below the pre-determinedminimum voltage level (which may correspond to or which may be below thelow voltage threshold 424), the information stored within the datastorage 504 may be lost.

When the supply voltage is turned on again, the remaining timing voltage422 (also referred to as the residual value of the timing voltage 422)at the timing circuit 401 provides an indication of the duration of theinterruption of the mains supply. The comparator 411 may be configuredto determine the remaining timing voltage 422. In particular, thecomparator 411 may be configured to determine whether the remainingtiming voltage 422 is greater or smaller than a pre-determined referencevoltage (e.g. the off-voltage threshold 424).

It should be noted that the ground node of the timing circuit 401 mayalso be e.g. a shunt of an external switch 212. This is illustrated inthe circuit arrangement 520 of FIG. 5 b which shows the timing circuit401 in conjunction with the external switch 212 of a switched-mode powerconverter 222 (see e.g. FIG. 2). The timing circuit 401 is arrangedbetween the source of the external switch 212 and the pin 216 of thecontroller 206. Furthermore, the timing circuit 401 may be coupled toground via the shunt resistor 215, wherein the shunt resistor 215 isindicative of the current through the external switch 212. By doingthis, the pin 216 of the controller 206 can be used for sensing thecurrent through the external switch 212 (during normal operation of thecontroller 206) and for sensing the timing voltage 422 (subsequent to aninterruption of the operation of the controller 206).

During normal operation, the external switch 212 alternates betweenon-state and off-state. When the external switch 212 is in off-state(and no current is flowing through the switch 212), the switch 521 maybe closed, thereby providing a pre-determined voltage level (e.g. 5V) atthe pin 216. As a consequence, the capacitor 402 of the timing circuit401 is re-charged such that the timing voltage 422 corresponds to thepre-determined voltage level. At the same time, the auto-zeroing switch523 may be closed, thereby (re-)charging the compensation capacitor 522to bring the compensation capacitor 522 to the pre-determined voltagelevel. When the external switch 212 is put into on-state, theautozeroing switch 523 is open, and the voltage drop at the shuntresistor 215 may be provided to the comparator 411 via the capacitor 402of the timing circuit 401 (referred to as the timing capacitor) and viathe compensation capacitor 522. The timing voltage 422 at the timingcapacitor 402 is compensated by the inverse voltage at the compensationcapacitor 522. As such, the comparator 411 may be used to supervise thecurrent through the external switch 212 during normal operation of thecontroller 206. In particular, during normal operation of the controller206, the comparator 411 may determine whether the current through theexternal switch 212 reaches or exceeds a pre-determined current peak.

On the other hand, the comparator 411 may be used to determine the levelof the timing voltage 422, upon resumption of operation of thecontroller 206. In this context, the reference voltage at the comparator411 may be modified. In particular, a different reference voltage may beused for measuring the current peak (during normal operation) and formeasuring the residual level of the timing voltage 422. Alternatively orin addition, the timing voltage 422 may be biased (using e.g. thebiasing circuit 407, 408 illustrated in FIG. 4), such that the samereference voltage may be used for measuring the current peak and formeasuring the residual level of the timing voltage 422.

It should be noted that the autozeroing switch 523 does not necessarilyhave to be coupled to ground. The autozeroing switch 523 may also becoupled to a different potential if the reference voltage of thecomparator 411 is selected accordingly. As such, the autozeroing switch523 may be coupled to any other potential than ground. The main functionof the autozeroing switch 523 (and the compensation capacitor 522) is toensure that the voltage of the capacitor 402 of the timing circuit 401is compensated, thereby providing an indication of the voltage drop atthe shunt resistor 215 at the comparator 411.

FIG. 6 illustrates a flow chart of an example method 600 for determiningan indication of the duration of an interruption of electrical energyprovided to a driver circuit 200 (e.g. an interruption of the mainssupply). The method 600 comprises the step of controlling 601 a powerconverter 221, 222 such that the power converter converts electricalenergy at an input voltage 230 into electrical energy at a drive voltage231. The controlling step 601 may be performed using the controller 206described in the present document. Furthermore, the method may comprisethe step of maintaining 602 a timing voltage 422 at a timing circuit 401above a first voltage level 423, when controlling the power converter221, 222. As outlined in the present document, the timing circuit 401 isseparate from the controller 206. Furthermore, the timing voltage 422 atthe timing circuit 401 typically decays with a pre-determined timeconstant if not maintained at or above the first voltage level 423 bythe controller 206.

The method may further comprise the step of stopping or interrupting 603controlling 601 of the power converter 221, 222 (e.g. due to aninterruption of the controller 206). As a result of the interruptedcontroller 206, maintenance 602 of the timing voltage 422 may bestopped. The interruption of the controller 206 and the resultinginterruption of the maintenance of the timing voltage 422 typicallyoccurs subsequent to (and as a result of) an interruption of electricalenergy provided to the driver circuit 200. The method 600 may proceed inresuming 604 controlling the power converter 221, 222. The controller206 may resume operation subsequent to (and as a result of) theprovision of electrical energy to the driver circuit 200. The provisionof electrical energy to the driver circuit 200 may be due to aninterruption of the mains supply (e.g. caused by the switching off of amains switch). The method may comprise the step of determining 605 anindication of a residual level of the timing voltage 422 when resuming604 the control of the power converter 221, 222. The indication of theduration of the interruption of electrical energy provided to the drivercircuit 200 may be determined 606, e.g. based on the first voltage level423, e.g. based on the pre-determined time constant and e.g. based onthe indication of the residual level of the timing voltage 422. Theindication of the residual level of the timing voltage 422 may e.g. bean indication whether the residual level of the timing voltage 422 isabove or below a pre-determined off-voltage threshold 424.

In the present document, a driver circuit comprising a timing circuitand a corresponding method have been described. The driver circuit isconfigured to determine the duration of an interruption of the mainssupply without the need for a stable supply voltage. The timing circuitis independent of the rest of the driver circuit. In particular, thetiming circuit is independent of the supply voltage provided by a supplyvoltage capacitor of the driver circuit. As a result, the driver circuitmay use relatively small supply voltage capacitors, thereby enabling ashort start-up time of the driver circuit. At the same time, the drivercircuit is enabled to measure the duration of an interruption of themains supply, thereby enabling the driver circuit for mains power switchcontrolled dimming applications.

It should be noted that the description and drawings merely illustratethe principles of the proposed methods and systems. Those skilled in theart will be able to implement various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples and embodiment outlined in the present document are principallyintended expressly to be only for explanatory purposes to help thereader in understanding the principles of the proposed methods andsystems. Furthermore, all statements herein providing principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass equivalents thereof.

What is claimed is:
 1. A driver circuit configured to provide electricalenergy at a drive voltage at an output of the driver circuit wherein theelectrical energy at the drive voltage is derived from electrical energyat an input voltage at an input of the driver circuit, the drivercircuit comprising: a power converter configured to convert theelectrical energy at the input voltage into electrical energy at thedrive voltage; a controller configured to control the power convertersuch that the power converter provides the electrical energy at thedrive voltage; wherein the controller is configured to stop operationsubsequent to an interruption of electrical energy provided at the inputof the driver circuit, and resume operation subsequent to the provisionof electrical energy at the input of the driver circuit; and a timingcircuit coupled to the controller, wherein a timing voltage at thetiming circuit decays with a pre-determined time constant if notmaintained; and wherein the controller is configured to maintain thetiming voltage at or above a first voltage level when the controller isin operation, determine an indication of a residual level of the timingvoltage when the controller resumes operation after an interruption ofelectrical energy provided at the input of the driver circuit, anddetermine an indication of the duration of the interruption ofelectrical energy provided at the input of the driver circuit based onthe first voltage level, the pre-determined time constant, and anindication of the residual level of the timing voltage.
 2. The drivercircuit of claim 1, wherein the timing circuit comprises a capacitorarranged in parallel to a resistor.
 3. The driver circuit of claim 1wherein the timing circuit couples a pin of the controller to ground. 4.The driver circuit of claim 1 wherein the power converter comprises apower switch, the controller is configured to control the power switchsuch that the power converter provides the electrical energy at thedrive voltage, the driver circuit comprises a shunt resistor arranged inseries with the power switch, such that a voltage drop at the shuntresistor is indicative of a current through the power switch, and thetiming circuit couples the controller to the shunt resistor.
 5. Thedriver circuit of claim 4, wherein the controller is configured tocouple the timing circuit with a comparator via a compensation capacitorand the compensation capacitor is configured to compensate the timingvoltage such that the voltage at the comparator corresponds to thevoltage drop at the shunt resistor.
 6. The driver circuit of claim 5,wherein the driver circuit comprises an autozeoring switch configured tocouple the compensation capacitor to a pre-determined potential whencharging the compensation capacitor, the compensation capacitor isconfigured to compensate the timing voltage such that the voltage at thecomparator corresponds to the voltage drop at the shunt resistor, andthe driver circuit comprises an auto zeroing switch configured to couplethe compensation capacitor to a pre-determined potential when chargingthe compensation capacitor.
 7. The driver circuit of claim 1 wherein thecontroller is configured to couple the timing circuit to a voltagesource thereby maintaining the timing voltage at or above the firstvoltage level.
 8. The driver circuit of claim 1, wherein the controlleris configured to couple the timing circuit with a comparator, thecomparator is configured to compare the timing voltage with a referencevoltage, and the comparator is configured to indicate whether the timingvoltage is greater or smaller than the reference voltage, therebyproviding an indication of the residual level of the timing voltage. 9.The driver circuit of claim 1 wherein the driver circuit is configuredto provide electrical energy at a drive voltage to a solid statelighting light source and the controller is configured to operate thepower converter according to a plurality of different operation statescorresponding to a plurality of different illumination states of thelight source.
 10. The driver circuit of claim 9, wherein the controlleris configured to operate the power converter according to a currentoperation state; detect one of a plurality of pre-determined eventsbased on the input voltage; determine a target operation state inaccordance to a pre-determined state machine, based on the currentoperation state and the detected one of the plurality of pre-determinedevents; and operate the power converter in accordance to the targetoperation state.
 11. The driver circuit of claim 10, wherein theplurality of pre-determined events comprises an “off” event and an“off/on”, detecting the off event comprises detecting that the residuallevel of the timing voltage lies below an off-voltage threshold, anddetecting the “off/on” event comprises detecting that the residual levelof the timing voltage lies above the off-voltage threshold.
 12. Thedriver circuit of claim 1 wherein the input voltage corresponds to arectified mains voltage.
 13. The driver circuit of claim 1 wherein thedriver circuit comprises a supply voltage capacitor coupled to thecontroller and configured to provide a supply voltage to the controller;the supply voltage capacitor is charged using electrical energy providedat the input of the driver circuit, subsequent to the interruption ofelectrical energy provided at the input of the driver circuit the supplyvoltage drops, the controller stops operation if the supply voltage isbelow a pre-determined supply voltage threshold; and the controllerresumes operation if the supply voltage subsequently increases above thepre-determined supply voltage threshold.
 14. The driver circuit of claim1 comprising an electrical connection module configured to connect amains power supply, thereby providing electrical energy at the drivevoltage to a light source.
 15. The driver circuit of claim 14 whereinthe light source comprises light emitting diodes.
 16. The driver circuitof claim 15 wherein the light emitting diodes are arranged in strings oflight emitting diodes wherein defects of individual light emittingdiodes can be either a short circuit or an open circuit.
 17. The drivercircuit of claim 16 wherein the strings of light emitting diodes areconnected in parallel.
 18. A method for determining an indication of theduration of an interruption of electrical energy provided to a drivercircuit, comprising: controlling a power converter to convert electricalenergy at a input supply voltage into electrical energy at a drivevoltage using a controller; providing a timing circuit, wherein thetiming circuit is separate from the controller; maintaining a timingvoltage at the timing circuit at a first voltage level, when controllingthe power converter, wherein the timing voltage at the timing circuitdecays with a pre-determined time constant if not maintained; stoppingcontrolling the power converter and stopping maintaining the timingvoltage subsequent to an interruption of electrical energy provided tothe driver circuit; resuming controlling the power converter subsequentto the provision of electrical energy to the driver circuit; determiningan indication of a residual level of the timing voltage when resumingcontrolling the power converter, and determining the duration of theinterruption of electrical energy provided to the driver circuit basedon the first voltage level, the pre-determined time constant, and theresidual level of the timing voltage.
 19. The method of claim 18 whereinthe timing circuit comprises a resistor aranged in parallel with acapacitor and the pre-determined time constant is the parallelresistor-capacitor time constant.
 20. The method of claim 18 whereindetermining the duration of the interruption of electrical energyprovided to the driver circuit is based the decay of the voltage of theparalles resistor-capacitor combination.
 21. The method of claim 18wherein determining the duration of the interruption of electricalenergy provided to the driver circuit is determined by the differencebetween the first voltage level and the residual voltage level of thetiming circuit and the pre-determined time constant.